This question came up a few years ago
http://www.bikeforums.net/archive/in.../t-218054.html
and
http://www.bikeforums.net/archive/in.../t-221412.html
At the time, Nashbar had a 1" JIS threadless headset (Aheadset I think) but it looks like they no longer do. Can you can mill your fork down and ream your head tube out for an ISO headset?
EDIT: Another possibility would be to install a 1" JIS threaded headset. Skip the keyed spacer and lock nut and just put on the threadless stem and top cap. You can probably pick up a 1" top cap at any bike shop if you don't already have one. You may need to ream out the threads on the top cup.